Optical processing system

ABSTRACT

A telecommunications system comprising first and second nodes interconnected by a network transmission line. The first node comprises an optical data generator for producing an optical data signal at a first wavelength, an optical header generator for producing an optical control signal at a second wavelength, and means for multiplexing the data and control signals onto the transmission line. The second node comprises a switch and a controller responsive to signals at the second wavelength for controlling the routing of optical signals through the switch. A delay unit and associated control means are provided to ensure that sufficient delay occurs between the transmission start times of the control and data signals that the control signal completely overlaps the data signal at the second node.

FIELD OF THE INVENTION

This invention relates to an optical processing system, and inparticular to optical header recognition in packet switching networks.

In a circuit switched telecommunications network, a physical circuit ismade between two terminals for the duration of a call. For certaintraffic formats, such as speech, the information being transmitted nevercompletely fills the connection between two terminals, that is to saythe start of the information does not reach the destination terminalbefore the end of that information leaves the transmitting terminals,but the circuit is held open for the duration of the information'stransmission between the two terminals. With high-speed circuitscarrying digital data, a much greater resource utilisation is possibleby sharing paths through the network. A packet switching network is oneway of achieving this improved resource utilisation, data beingtransferred through the network in packets. Each packet includes, apartfrom the data itself, a header containing addressing and sequence(control) information for controlling the progress of that packetthrough the network. The addressing and sequence information coded inthe header of a packet is decoded at the network nodes to providerouting control. A packet switching network thus provides a virtualcircuit between two terminals, this circuit appearing to the users as apermanent connection between the terminals but which, in fact, is sharedwith other users.

BACKGROUND OF THE INVENTION

Known methods of coding packet headers rely on time correlationtechniques. The utilisation of a packet switching network is linked tothe bit-rate. The utilisation also depends upon the ratio of data timeto wasted time, that is to say the ratio of the time the network istransmitting data to the time data is not being transmitted. In the timedomain, the wasted time is made up of the time taken up with headertransmission (the header of a packet occupying a separate time slot atthe head of the data of that packet), and by the guard band transmissiontime, the guard band being the separation between adjacent packets whichis essential to avoid overlap of the packets due to dispersion duringtransmission.

BRIEF DESCRIPTION OF THE INVENTION

The aim of the invention is to provide an alternative technique forcoding and decoding header information, particularly in packet switchingnetworks, which technique results in increased network utilisation.

The present invention provides a telecommunications system comprisingfirst and second nodes interconnected by a network transmission line,the first node comprising an optical data generator for producing anoptical data signal at a first wavelength, an optical header generatorfor producing an optical control (header) signal at a second wavelength,means for multiplexing the data and control signals onto thetransmission line in such a manner that the duration of the controlsignal is at least equal to the duration of the data signal, a delayunit and control means associated with the delay unit for providing asufficient delay between the transmission start times of the control anddata signals to ensure that the control signal completely overlaps thedata signal on arrival at the second node, the second node comprising aswitch and a controller responsive to signals at the second wavelengthfor controlling the routing of optical signals through the switch.

As the control signal overlaps the data signal, the two signals occupythe same time slot.

Advantageously, the optical data generator produces optical data signalsin packets, and preferably the optical data generator is constituted bya laser and a modulator for modulating the output of the laser. Theheader generator may also be constituted by a laser.

The system may further comprise a modulator for modulating the headerlaser so as to turn the header laser on at, or just before, the start ofa data packet and to turn the header laser off at, or just after, theend of a data packet.

Conveniently, the controller of the second node includes a splitter fordemultiplexing a portion of the control signal, and a narrow band-passfilter whose pass band is centred on the second wavelength, the outputof the filter being used to control the operation of the switch. Anamplifier may be positioned between the splitter and the filter.

Advantageously, the switch is an optical switch such as a NLOA.Alternatively, the switch is an opto-electronic switch. In either case,the switch may have two outputs, one of which leads to a further networktransmission line, and the other of which leads to a receiver.Preferably, a narrow band-pass filter is positioned between the switchand the receiver, the pass band of said filter being centred on thefirst wavelength.

Preferably, there is a plurality of second nodes, the nodes beinginterconnected by network transmission lines, and the optical datagenerator and the optical header generator of the first node are tunableso as to provide data and control signals at predetermined, differentwavelengths for each of the second nodes.

In a preferred embodiment, the or each second node is provided with amodule for injecting data and control signals onto a transmission line.Advantageously, the or each module comprises an optical data generatorfor producing an optical data signal at a first predeterminedwavelength, an optical header generator for producing an optical controlsignal at a second predetermined wavelength, and means for multiplexingsaid data and control signals onto a transmission line.

Preferably, the or each module further comprises a memory store forstoring data awaiting transmission, and control means and look-up tablesfor determining the first and second predetermined wavelengthsappropriate to the required destination of the signals being injected.

Advantageously, the optical data generator, the optical header generatorand the multiplexing means of the first node are incorporated into amodule, said module further comprising a memory store for storing datawaiting transmission, control means and look=up tables for determiningthe wavelengths of the control and data signals appropriate to thedestination node of the signals being injected. In this case, the firstnode may include a switch and a controller responsive to signals at apredetermined wavelength for controlling the routing of optical signalsthrough the switch, said controller including a splitter fordemultiplexing a portion of an incoming control signal and a narrowband-pass filter whose pass band is centred on said predeterminedwavelength, the output of the filter being used to control the operationof the switch.

A respective delay unit may be associated with the control means and thelook-up tables of each module for providing a sufficient delay betweenthe transmission start times of the control and data signals to ensurethat the control signal completely overlaps the data signal on arrivalat a destination node.

Advantageously, the modules are provided with additional control meansfor adjusting the look-up tables to compensate for changes in theeffective optical path length of inter-node network transmission lines.Preferably, the additional control means of each module is constitutedby first and second processing means, the first processing means beingeffective to monitor incoming control signals and to feed back opticalpath length information derived therefrom to the node transmitting saidcontrol signals, and the second processing means being associated withthe look-up tables of that module to up-date said look-up tables independence upon optical path length information received from the firstprocessing means of another module. Conveniently, the first processingmeans of each module is a local processor associated with the controlmeans of that module, and the second processing means is constituted bylocal processors associated with the look-up tables of that module. Thesystem may further comprise a management centre for controlling thelocal processors.

DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a wavelength headercoding/decoding apparatus constructed in accordance with the invention;

FIG. 2 is a schematic representation of one form of optical switch whichcould be used in the apparatus of FIG. 1;

FIGS. 3a to 3d illustrate the output signal behaviour of the opticalswitch of FIG. 2;

FIG. 4 is a schematic representation of a simple ring networkincorporating apparatus of the type shown in FIG. 1;

FIG. 5 is a schematic representation of a star network incorporatingapparatus of the type shown in FIG. 1;

FIG. 6 is a schematic representation of a ring/star networkincorporating apparatus of the type shown in FIG. 1; and

FIG. 7 is a schematic representation of a wavelength routing networkcrossconnect switch for use with the networks of FIGS. 5 and 6.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to the drawings, FIG. 1 shows one node 1 of a packet switchingoptical fibre network, the network including a plurality of similarnodes. The node 1 is connected to the network via input and outputfibres 2 and 3 respectively. The input fibre 2 is connected to ahead-end station (not shown in detail) provided with an optical datagenerator 4 and a header generator 5. The optical data generator 4produces data packets of 16-bit length (one of which is shown at 4a) bymodulating a laser (not shown) at 2.5 Gbit/sec and at a wavelength of1.55 μm. The header generator 5 produces header (control) signals (oneof which is shown at 5a) by modulating a second laser (not shown) at aneffective rate of 155 Mbit/sec corresponding to data packets of 16-bitlength, and at a wavelength of, for example, 1.3 μm. This modulation ischosen so that the laser of the header generator 5 is turned on at, orjust before, the start of a data packet 4a, off at, or just after, theend of that data packet. The control signal wavelength is chosen tomatch the receive wavelength of the node 1, and the header generator 5is tunable so as to provide control signals at different wavelengths,each of which matches the receive wavelength of another network node.The two signals 4a and 5a are superimposed onto the fibre 2 by means ofa WDM coupler 6.

The node 1 includes a four-port optical switch 8 for adding data to, anddropping data from, the network. The switch 8 has first and second inputports 9a and 9b respectively, the first input port being connected tothe input fibre 2 via a splitter 7, and the second input port beingconnected to a data add module 10 (to be described in greater detailbelow). The switch 8 has first and second output ports 11a and 11brespectively, the first output port being connected to the output fibre3, and the second output port being connected to a 2.5 Gbit/sec receiver12 via a band-pass filter 13.

The splitter 7 demultiplexes a small proportion (typically a fewpercent) of the control signal 5a of an incoming packet, and feeds thistapped signal to a band-pass filter 14 via a 1.3 μm optical amplifier15. The filter 14 has a narrow pass band centred on 1.3 μm, so that itwill pass the tapped signal provided the wavelength of the tapped signalmatches that of the pass band of the filter. The output of the filter 14is fed to a control port 16 of the optical switch 8, thereby to open theswitch and connect the first input port 9a to the second output port11b. In this way, a data packet intended for the node 1 is dropped toits receiver 12. As the control signal 5a overlaps the data signal 4a inthe packet, the switch 8 is opened at, or just before, the start of thedata reaches the switch and is closed at, or just after, the end of thedata leaves the switch. Thus the control signal applied to the controlport 16 has at least the same time duration as the data packet. Thefilter 13 has a narrow pass band centred on a wavelength of 1.55 μm (thedata wavelength), so that the signal reaching the receiver 12 is solelya data signal. The filter 13 not only filters out the remaining controlsignal 5a, it also filters out noise. If the wavelength of the tappedsignal does not match that of the pass band of the filter 14, the filterhas no output signal and the optical switch 8 remains closed, that is tosay its first input port 9a is connected to its first output port 11a.In this way, the data/control packet associated with the tapped signalis routed through the node 1 to the output fibre 3 and on into thenetwork.

The switch 8 is preferably an all-optical switch such as a non-linearoptical amplifier (NLOA). Alternatively, the switch could be anopto-electronic device such as a lithium niobate switch, in which casean opto-electronic converter 17 (shown in dashed lines) would beincluded in the path between the filter 14 and the control port 16 ofthe switch 8. The converter 17 would not require any processingcapabilities, but would need to carry out a certain amount ofamplification to ensure that a sufficiently large electronic signal isinput to control the switch 8. Simple opto-electronic components of thistype are readily available; and, combined with known switchingtechnology, can produce switch rise and fall times of much less than 1ns.

The module 10 of the node 1 can add data packets onto the network wheneither a packet has been dropped by the node (having been triggered bythe header address decoder described above), or if some protocol (suchas a token-ring type protocol) allows input onto an empty line whilstensuring controlled and fair network access. Data packets fortransmission in this way are held in a memory store 18 provided in themodule 10. The module 10 also includes an optical header generator 21and a data generator 20. The generators 20 and 21 are tunable so as totransmit data at any one of a plurality of predetermined wavelengths,and to transmit control signals at any one of a plurality of differentwavelengths. Respective look-up tables 22 and 23 are associated with thedata and header generators 20 and 21 respectively, so that thewavelengths of both the data and the control signals for the requireddestination of a given packet are correctly provided. If dispersion is apotential problem, the look-up tables 22 and 23 can work out thedifference between the transmission times of the control signal and thedata signal so chosen, and can instruct a delay unit 24 to provide anappropriate delay between the transmission start times of the controland data signals, thereby to ensure that the control signal 5acompletely overlaps the data signal 4a at the destination node, therebyensuring that its optical switch 8 routes the whole of the data signaland does not lose any data bits. The loss of data bits would causeerrors, and thereby detract from the operational characteristics of thenetwork.

The head-end station also includes a memory store, look-up tables and adelay unit (similar to the items 18, 22, 23 and 24 of the module 10), sothat data for transmission can be held awaiting transmission, the dataand header wavelengths for transmission to any given node of the networkcan be worked out, and an appropriate delay can be provided in thetransmission store times in the header and data signals for reducingdispersion problems. Indeed, the head-end station may include a data addmodule of the same type as that provided at the node 1. It would also bepossible to provide the node 1 (and any other similar node connected tothe network) with a tunable filters 13 and 14 so that the wavelengths ofthe control and data signals appropriate to each of the nodes can bealtered, for example by a management centre, if required. In this case,it would be possible for the head-end station to be identical to each ofthe nodes 1 in the network.

Because of changes in the effective optical path length of network linkscaused by environmental alterations, such as temperature, the delaybetween any pair of nodes alters. This alteration could result in lossof information at the destination node 1, due to the control signal 5amoving with respect to the data signal 4a. This change in optical pathlength will probably only occur on time scales no greater than the kHzlevel. In order to ensure that all the optical path lengths are knownand that the network remains "synchronized" (that is to say the controlsignals 5a overlap the data signals 4a at all the nodes 1), feedbackinformation between the nodes is needed to monitor optical path lengths,and to adjust the look-up tables 22 and 23 accordingly. This feedbacksignal can be achieved by monitoring the arrival of the control signals5a at the nodes 1. Thus, if the network knows where the information hascome from (by monitoring the fibre that the signal arrived on), andmonitors the relative time that a given control signal 5a is incident onthe node, then any differences in path delay can be monitored. As shownin FIG. 1, this monitoring can be achieved by providing the look-uptables 22 and 23 of each node 1 with local processors 22a and 23a, andby tapping off a small percentage of the output signal of the filter 14of each node 1 to a further local processor 14a. The processor 14a of adestination node 1 determines what has happened to the network, andsends a suitable update control signal through the network to all theother nodes to tell them how to update their look-up tables 22 and 23.These update signals may go via a central management centre (not shown)provided at the head-end station, or via some other management centre,perhaps linking a sub-set of the nodes 1. The need to provide amanagement centre depends on whether the total processing time of thelocal processors 22a, 23a and 14a is sufficient to make sure that thenetwork stays "stable", and that the update control signals do not causeproblems by changing the network after it has naturally recovered to itsnormal state (or it is still responding to previous signals). In otherwords, the time taken to adjust the network should be at most equal tothe time-constants of the perturbing effects.

The local processors 22a and 22a in each of the transmission nodes 1receive update control information from every other node in the network,and process this to modify their associated look-up tables 22 and 23correctly. Thus, the look-up tables 22 and 23 of all the nodes 1(including the head-end station) are continually up-dated to compensatefor environmental alterations. The degree of intelligence that the localmanagement processors 22a and 23a have will dictate the strategy for thelook-up table upgrade. Thus, it would be ideal if the processors 22a and23a look at a number of inter=dependent signals to work out the bestsolution for the network as a whole, covering all the links that theinformation traverses on its way to a given destination. The capabilityneeded is, therefore, related to the number of nodes 1 in the network.

The viability of the coding/decoding apparatus (and in particular theviability of using an NLOA as the optical switch 8) described above withreference to FIG. 1 has been tested experimentally using theconfiguration shown in FIG. 2. A data signal 4a at a wavelength of about1.55 μm (1.535 μm to 1.56 μm operational range) was modulated at 1 Gb/sto 2.5 Gb/s. The control signal 5a was a 1.31 μm DFB laser modulatedwith 1010 pattern at 1/16th the bit-rate of the data. These signals 4a,5a were injected into the absorber facet 8a of a bulk material NLOA 8which was under standard bias conditions. Improved performance occurredwhen the absorber bias was reduced. The output from the NLOA 8 wasfiltered using a band-pass filter 13 at the data wavelength. Typicalgated data signals are shown in FIGS. 3a to 3d. These results are for 1Gb/s data, but identical behaviour was observed at speeds of 2.5 Gb/s.NLOAs operating at >5 Gb/s have been demonstrated, and further speedimprovements are expected with device optimisation.

Operation in two modes (resonant amplifier and injection locked) hasbeen demonstrated. The resonant amplifier mode results are shown inFIGS. 3a and 3b, while those for the injection locked case are shown inFIGS. 3c and 3d. The measured extinction ratio for both cases was >13 dBbetween the gated data and the rejected data signals, and the EYEdiagram shows a clean opening and good error-ratio performance isexpected. The contrast ratio (the on-level power relative to theoff-level power referenced to 0) was >10 dB. The rise and fall of thegate for the resonant amplifier case was ˜2-5 ns, and dependent on thedetuning of the data wavelength from the NLOA Fabry-Perot mode. Adetuning range of ˜10 GHz was possible which would require wavelengthreferencing in a network configuration to ensure good performance.

In the injection locked mode (NLOA almost or at threshold), the rise andfall times were less than a bit-period (400 ps), but the networkbenefits of this faster gating time are balanced by a much tighterdetuning requirement, with successful operation obtainable over a datawavelength range of approximately 1-2 GHz.

The technique described above can be used in packet, virtual and circuitsystems. It maintains a transparent data channel, and puts the necessarybit-rate specific information (such as packet duration, required riseand fall times etc) into a control channel at a different wavelength.The principle of the invention could also be used in "frame" systems,such as synchronised digital hierarchy (SDH) where the data bit-rate isset, and to fast circuit switched networks. The technique could also beused for distribution applications for data communications networks inLAN, MAN and WAN environments, and the general principle may also beused in trunk applications if configured correctly.

The technique can be used in ring, start and star/ring topologies asdescribed below with references to FIGS. 4 to 6. Thus, FIG. 4 shows onepossible configuration for using the wavelength header coding/decodingtechnique of the invention in a simple ring network. This networkincludes four nodes 31, each of which is similar to the node 1 ofFIG. 1. The nodes 31 are connected in a ring configuration at the end ofa trunk spur 32. Each of the nodes 31 has a different address wavelengthλ₁, λ₂, λ₃ and λ₄ which matches the control signal wavelengths input bythe trunk spur 32. Obviously, therefore, the filters 14 of the nodes 31are different, each having a narrow pass band centred on the appropriateaddress wavelength λ₁, λ₂, λ₃ or λ₄.

Data from the network enters the ring via the trunk spur 32 and a trunkmultiplexer (such as a 3 dB coupler) 33, and travels around the ringreaching each of the nodes 31 in turn. At each node 31, the informationon the line is interrogated and, when the control signal 5a of any givenpacket matches the address wavelength of a node, the data is routed offthe ring, and local data ready for transmission into the network can beadded in its place. As with the embodiment of FIG. 1, there is anadd/drop function at the wavelengths of both data and control signals.Data circulating in the ring is multiplexed back onto the trunk spur 32after travelling completely around the ring. This type of configurationcould, therefore, be useful for signalling networks with the transfer ofcontrol information between nodes.

Information entering and leaving the ring does not necessarily need tobe at the same wavelength or bit-rate if the trunk multiplexers aredesigned correctly. For example, if the trunk network is a wavelengthrouted network (at the data wavelength) then outward information can betransmitted at any of the available network wavelengths. The controlsignal wavelengths can, therefore, also be any convenient value.Although FIG. 4 shows only four nodes 31 on the ring, it will beapparent that the principle can be extended to virtually any number ofnodes, this number being dictated by factors such as the controlwavelength range, the filter bandwidth, the pass bandwidth of thewavelength-routed cross-connects elsewhere in the network, and anydispersion problems. As mentioned above, each of the nodes 31 includesan amplifier for amplifying the tapped signal, so that a very lowpercentage of an input signal needs to be tapped, so that many nodes canbe concatenated.

FIG. 5 shows a star topology network having five rings 40 each of whichis similar to the ring described above with reference to FIG. 4. Eachring 40 includes four nodes 41, each of which is similar to the node 1of FIG. 1. Each of the rings 40 is connected to a wavelength routedcross-connect 43 via a respective trunk spur 42. Each of the trunk spurs42 is arranged to carry data at a respective data wavelength λ_(data1),λ_(data2), λ_(data) 3, λ_(data4) and λ_(data) 5. Each of the nodes 41 ofeach ring 40 has different address wavelength λ₁ to λ₂₀ which matchesthe header wavelengths input by the trunk spurs 42. Here again, thefilters 14 of the nodes 41 are different, each having a narrow pass bandcentred on the appropriate address wavelength λ₁ to λ₂₀.

The wavelength routed cross-connect 43, which interconnects the fiverings 40, ensures that the control signals are always routed over thesame effective path as the associated data. This cross-connect 43 isshown in detail in FIG. 7, and has the same interconnections for bothcontrol and data fields, any switching within these fields being drivenin synchronism. A node 41 that wants to transmit data to another node 41within the network choses the correct data wavelength (for exampleλ_(data1)) and the correct control signal wavelength (for example λ₂).The cross-connect 43 is designed to route control signal bands ratherthan single wavelengths, that is to say a band of wavelengths λ₁ to λ₄is routed rather than routing each of these wavelengths separately. Thisprinciple could also be used to route the data wavelengths, which wouldincrease the capacity of the network.

FIG. 6 shows a star-ring topology having five rings 50, each of whichincludes four nodes 51, each being similar to the node 1 of FIG. 1. Eachof the rings 50 is connected to an inner ring 54 via a respective trunkspur 52 and a wavelength routed cross-connect 53. The trunk spurs 52also lead to a central wavelength routed cross-connect 55, and each isarranged to carry data at a respective data wavelength λ_(data1) toλ_(data5). Each of the nodes 51 of each ring 50 has a different addresswavelength λ₁ to λ₂₀ which matches the control signal wavelengths inputby the trunk spurs 52. Here again, the filters 14 of the nodes 51 aredifferent, each having a narrow pass band centred on the appropriateaddress wavelength λ₁ to λ₂₀. The wavelength routed cross-connects 53and 55 are similar to that shown in FIG. 7, and ensure that the controlsignals are always routed over the same effective path as the associateddata.

Any switching that is required, for contention resolution or re-routing,at any of the cross-connects 43, 53 and 55 will require that an arrivingcontrol signal 5a completely overlaps in time its associated data signal4a. This overlap need only occur within a given switching window. Thecontrol of the look-up tables 22 and 23 of the transmitting node willneed to take this into account when setting up the transmission. Thecomplexity of the network and the choice of wavelengths is, therefore,related. This is particularly the case where packets are routed throughone or more cross-connects between a transmission node and a destinationnode, where it may be essential to ensure overlapping of control anddata signals at the cross-connect(s)--although this may not be requiredif the cross-connects are such that the optical switch within thecontrol cross-connect can operate non-synchronously with respect to theoptical switch within the data cross-connect.

I claim:
 1. A telecommunications system comprising first and secondnodes interconnected by a network transmission line, the first nodecomprising an optical data generator for producing an optical datasignal at a first wavelength, an optical header generator for producingan optical control signal at a second wavelength, means for multiplexingthe data and control signals onto the transmission line in such a mannerthat the duration of the control signal is at least equal to theduration of the data signal, a delay unit coupled with the optical datagenerator and the optical header generator and control means coupledwith the delay unit for providing a sufficient delay between thetransmission start times of the controller and data signals to ensurethat the control signal completely overlaps the data signal on arrivalat the second node, the second node comprising a switch and a controllercoupled to the transmission line, said controller being coupled to saidswitch and responsive to signals at the second wavelength forcontrolling the routing of optical signals through the switch.
 2. Asystem as claimed in claim 1, wherein the optical data generatorproduces optical data signals in packets.
 3. A system as claimed inclaim 1, wherein the optical data generator is constituted by a laserand a modulator for modulating the output of the laser.
 4. A system asclaimed in claim 1, wherein the header generator is constituted by alaser.
 5. A system as claimed in claim 4, further comprising a modulatorfor modulating the header laser so as to turn the header laser on at, orjust before, the start of a data packet and to turn the header laser offat, or just after, the end of a data packet.
 6. A system as claimed inclaim 1, wherein the controller of the second node includes a splitterfor demultiplexing a portion of the control signal and a narrowband-pass filter whose pass band is centred on the second wavelength,the output of the filter being used to control the operation of theswitch.
 7. A system as claimed in claim 6, further comprising anamplifier positioned between the splitter and the filter.
 8. A system asclaimed in claim 1, wherein the switch is an optical switch.
 9. A switchas claimed in claim 8, wherein the optical switch is a NLOA.
 10. Asystem as claimed in claim 1, wherein the switch is an opto-electronicswitch.
 11. A system as claimed in claim 1, wherein the switch has twooutputs, one of which leads to a further network transmission line, andthe other of which leads to a receiver.
 12. A system as claimed in claim11, further comprising a narrow band-pass filter positioned between theswitch and the receiver, the pass band of said filter being centred onthe first wavelength.
 13. A system as claimed in claim 1, wherein thereis a plurality of second nodes, the nodes being interconnected bynetwork transmission lines, and the optical data generator and theoptical header generator of the first node are tunable so as to providedata and control signals at predetermined, different wavelengths foreach of the second nodes.
 14. A system as claimed in claim 1, whereinthe or each second node is provided with a module for injecting data andcontrol signals onto a transmission line.
 15. A system as claimed inclaim 14, wherein the or each module comprises an optical data generatorfor producing an optical data signal at a first predeterminedwavelength, an optical header generator for producing an optical controlsignal at a second predetermined wavelength, and means for multiplexingsaid data and control signals onto a transmission line.
 16. A system asclaimed in claim 15, wherein the or each module further comprises amemory store for storing data awaiting transmission.
 17. A system asclaimed in claim 15, wherein the or each module further comprisescontrol means and look-up tables for determining the first and secondpredetermined wavelengths appropriate to the required destination nodeof the signals being injected.
 18. A system as claimed in claim 13,wherein the optical data generator, the optical header generator and themultiplexing means of the first node are incorporated into a module,said module further comprising a memory store for storing data waitingtransmission, the control means and look-up tables for determining thewavelengths of the control and data signals appropriate to thedestination node of the signals being injected.
 19. A system as claimedin claim 18, wherein the first node includes a switch and a controllerresponsive to signals at a predetermined wavelength for controlling therouting of optical signals through the switch, said first nodecontroller including a splitter for demultiplexing a portion of anincoming control signal and a narrow band-pass filter whose pass band iscentred on said predetermined wavelength, the output of the filter beingused to control the operation of the first node switch.
 20. A system asclaimed in claim 17, further comprising a respective delay unitassociated with said control means and said look-up tables of eachmodule for providing a sufficient delay between the transmission starttimes of the control and data signals to ensure that the control signalcompletely overlaps the data signal on arrival at the destination node.21. A system as claimed in claim 18, wherein the modules are providedwith additional control means for adjusting the look-up tables tocompensate for changes in the effective optical path length ofinter-node network transmission lines.
 22. A system as claimed in claim21, wherein the additional control means of each module is constitutedby first and second processing means, the first processing means beingeffective to monitor incoming control signals and to feed back opticalpath length information derived therefrom to the node transmitting saidcontrol signals, and the second processing means being associated withthe look-up tables of that module to up-date said look-up tables independence upon optical path length information received from the firstprocessing means of another module.
 23. A system as claimed in claim 22,wherein the first processing means of each module is a local processorassociated with the control means of that module, and the secondprocessing means is constituted by local processors associated with thelook-up tables of that module.
 24. A system as claimed in claim 23,further comprising a management centre for controlling the localprocessors.